Hot melts are in solid phase at ambient temperatures, but exist in liquid phase at elevated operating temperatures in inkjet printing devices. At the inkjet operating temperatures droplets of liquid hot melt are ejected from the printing device and, when the droplets contact a surface of a printing material, they harden to form a predetermined pattern of droplets.
Hot melts have been employed in direct and transfer printing processes. Hot melts are typically cast into solid sticks and placed into an inkjet printing device. The temperature of the inkjet device is raised to an operating temperature where a liquid phase with selective fluid properties is formed. The hot melt is then held as a liquid at the operating temperature in a reservoir and printhead of the inkjet printer. The hot melt in its liquid phase may then be applied in a predetermined pattern onto a substrate. While hot melts have been used for some time in the conventional printing industry, the electronics industry is beginning to appreciate the potential use of such compounds to address the problems in the manufacture of electronic devices, such as in the manufacture of printed circuit boards (PCBs).
PCBs are typically made by complex processes such as with dry film negative photoresist processes involving six or more stages. Firstly, a dielectric substrate is laminated or coated with copper and the copper surface is then overlaid with a photoresist layer. A photo-tool is prepared which is a negative of the required electrically conductive circuitry of the printed circuit. The photo-tool is placed directly over the photoresist layer to polymerise and harden in those areas exposed to the UV light to produce a latent image of the required electrically conductive circuitry in the photoresist layer. The photoresist layer is then developed to remove the unexposed area of the photoresist. This chemical treatment is typically mildly alkaline where the photoresist layer contains free carboxylic groups.
The exposed copper is then selectively removed by chemical etching from those areas not protected by the photoresist layers. Finally, the exposed areas of the photoresist layer are removed chemically, for example using stronger aqueous alkali where the photolayer contains free carboxylic acid groups.
Although the process is widely used in the manufacture of PCBs it is tedious, expensive and wasteful of materials since the photoresist layer is made separately and applied over the total area of the copper/dielectric substrate laminate. Furthermore, the photo-tool containing the negative image of the desired electrically conductive circuitry is often distanced from the photo-tool layer such that diffraction of UV light irradiation occurs and leads to development and polymerization in areas of the photoresist not directly beneath the UV transparent areas of the photo-tool. Such problems must be taken into consideration when preparing photo-tools and may reduce the density and definition of the electrically conductive circuitry. Furthermore, the chemical structure of the photoresist must be carefully controlled since its removal both before and after exposure to UV light depends on the alkaline treatment. The density and integrity of the intended electrically conductive circuitry can be seriously compromised if either the unexposed photoresist is incompletely removed or if some of the exposed and polymerized photoresist is removed prior to chemically etching the copper. Accordingly, there exists significant attraction in applying photoresist or similar materials to specific areas of a copper/dielectric laminate using inkjet printing technology.
When inkjet printing is done using hot melt inks, the image or negative image is made digitally available direct from a computer, the number of process steps is halved, and the need for differential removal of the hot melt ink using mild aqueous alkali is avoided. Also, since there is no photo-tool which is distanced from the hot melt ink layer, there is a potential for improved definition and density of the circuitry. There also exists the cost savings in terms of hot melt ink material since the hot melt is only applied to those areas to be protected from chemical etching.
Substantially complete removal of hot melt materials is highly desirable. If hot melt residue is left on a substrate after removal, the residue may compromise further processing of the substrate. For example, hot melt inks may be deposited on a PCB to function as a negative mask for forming a circuit pattern. Sections of the substrate which are not covered by the mask are etched away using an etchant and the hot melt ink is then stripped. Subsequent steps typically involve one or more metal plating processes. Any residue remaining on the PCB after stripping may compromise metal plating resulting in a defective electronic device.
A major problem which often arises during formation of the circuitry is undercutting. This results in defective and inefficient PCBs. This problem is common when the circuits are formed using an etching method in combination with a mask. Upon application of the etch to the selectively masked substrate the etch may not only remove portions of the substrate not covered by the mask but by capillary action seep under the mask at the interface of the mask and the substrate causing portions of the substrate covered by the mask to be undesirably etched away. This results in circuitry having irregular widths which results in irregular and non-uniform current flow. In addition, such undercutting may form tributaries which adjoin adjacent current lines resulting in electrical shorts.
Ammoniacal etchant which is a mixture of ammonia and ammonium chloride, is a high alkaline etching solution typically used in the outer layer circuitry fabrication of PCBs as opposed to inner layers. Most alkaline etch baths are designed to work in a pH range of 7.8 to 8.9; however, higher pH is preferred for a faster etch rate to reduce the contact time between the etchant and the PCB thus to reduce the opportunity of spreading of the etchant to other parts of the PCB and attacking the other parts which are sensitive to high pH values. Although most inkjet printable inner layer etch resist formulations are effective at a low pH range, inner layers are very sensitive to high pH values of 8.5 or greater. Etching at high pH can cause chemical attack of some etch resists resulting in an unacceptable large undercut or lifting off of layers from the substrate caused by a combination of capillary undercut of the etching chemistry and etchants penetrating into the resist causing swelling and adhesion failure.
As the industry seeks to manufacture electrical devices using thinner and more delicate circuitry and at the same time increase the plurality of circuits to increase electrical out-put, the foregoing problems become compounded by difficulty of working with smaller and more delicate materials. Accordingly, there is a need for a method which substantially reduces or eliminates the problem of undercutting in the formation of electrical circuits and improves stripping of the resists from substrates.